A plasma display screen is a display screen of the matrix type, formed from cells disposed at the intersections of rows and columns. A cell comprises a cavity, filled with a noble gas, and at least two control electrodes. In order to create a light point on the screen using a given cell, the cell is selected by applying a potential difference between these control electrodes, then ionization of the gas in the cell is triggered generally by means of a third control electrode. This ionization is accompanied by an emission of ultraviolet rays. The creation of the light point is obtained by excitation of a red, green and blue light-emitting material, by the emitted radiation. Conventionally, the control of a plasma display screen essentially comprises two phases, namely an addressing phase in which the cells (pixels) that will need to be lit and those that will need to be extinguished are determined, together with a display phase proper in which the cells having been selected in the addressing phase are effectively lit.
The addressing phase comprises a sequential selection of the rows of the matrix. By way of example, the rows not selected are set at a standby potential, for example 150 volts, whereas a selected row is set at an activation potential, for example 0 volts. In order to select chosen pixels from the selected row, pixels that will need to be lit in the display phase, the corresponding columns of the matrix are, for example, raised to a relatively high potential, for example 70 volts, by means of a power stage comprising MOS power transistors. The columns corresponding to the other pixels of the selected row, which will not need to be lit, are set at the potential 0 volts. Thus, the cells of the activated row, which will need to be lit, see a column-row potential equal to around 70 volts, whereas the other cells of this row see a column-row potential equal to 0 volts.
However, it may also be envisioned in the addressing phase, with the application of different potentials to the rows of the matrix, that a high potential be applied to a column in order to select a pixel that will need to be extinguished, and to apply a low potential to a column in order to select a pixel that will need to be lit.
The International Patent Application WO 02/15 163 gives an example of the general operation of such a plasma display screen, and focuses in particular on the problem of the selection of the columns when a row has been selected. More precisely, this document of the prior art states and solves the problem of the current spike flowing through the power transistors connected to the selected row, when a very large number of columns are simultaneously selected (corresponding to a very large number of pixels that are to be lit).
Aside from the problem of the current spike flowing through the power transistors when a very large number of columns are simultaneously selected, the inventors have identified another problem in the control of the cells of a plasma display screen, more particularly when the control is affected with a signal comprising a transition from a low potential to a high potential.
More precisely, the case can be taken where all the pixels of the row i must be in a given first state, for example lit (or extinguished, depending on the chosen convention) and all the pixels of the following row i+1 must be in the other state, for example extinguished (or lit, depending on the convention envisaged). In this case, in the addressing phase, when the row i is selected, all the columns of the display receive a command to change state, in other words their potential will be raised to a high state (to 70 volts for example), by following a rising edge over a given time. Subsequently, when the next row, i+1, is activated, the potential on the columns must be made to fall back to a low state (0 volts for example).
The transition from one state to another is carried out by applying a logic signal to a driver module situated on each of the columns in such a manner that one of the power transistors of the driver module is turned on in order to allow the capacitance of the cell in question to charge up or to discharge (depending on the convention considered). It has been observed that when a very large number of columns go from a low potential to a high potential, for example at least two thirds of the display columns, the rising edges of the respective column voltages are especially steep, in other words the rise time is around 40 nanoseconds. This leads to an emission of additional electromagnetic interference that can affect the operation of other components situated in close proximity.
Furthermore, the value of the selection current may vary from one circuit to another depending on the value of the load connected to the selection circuit. In particular, when the value of the load is relatively low, the selection current can take a very high value resulting in a strong over-voltage spike within the voltage source supplying the high potential (here 70 volts) for activation of the display column driver circuit.
The International Patent Application WO 02/41 292, assigned to the present applicant, proposes a solution where the column driver circuit comprises a current source so as to control the voltage applied to the gate of the power transistor delivering the selection signal. The control will be effective during the entire rise time of the transition signal from its low value to its high value. The rise time of the selection signal is therefore fixed and constant, and the value of current, delivered by the controlled power transistor, will be limited over the whole transition.
However, the integration of a current source into the column driver circuit represents a high cost in terms of the silicon surface area required to fabricate the circuit. In addition, the steep edge of the selection signal is attenuated by the control affected by the current source but on the contrary, at the end of the transition, the latter is steeper than it was, which generates high-frequency signals that promote electromagnetic emissions. Furthermore, the current source is sensitive to the variations in the driver circuit power supply voltage, which interferes with the control effected on the gate of the power transistor. This method aims to provide a solution to these problems.